The invention relates to a switching device for operating a lamp by means of a high-frequency current, and more particularly, to a switching device for preheating and igniting a lamp,
a control circuit which is coupled to a control electrode of the switching element, which control circuit comprises a secondary winding L2 of the transformer T and a timer circuit which is coupled to the secondary winding L2 and which is provided with a series arrangement of a resistive impedance R1, a capacitive element C1 and an auxiliary switching element S2, which is coupled to a junction point of resistive impedance R1 and capacitive element C1 and to a control electrode of the switching element S1.
Such a switching device is known from U.S. Pat. No. 4,525,648. The DC-AC converter in the known switching device is of the half-bridge type and the branch A comprises two switching elements which are alternately rendered conducting and non-conducting via respective secondary windings of the transformer T. Each switching element is coupled to a control circuit which comprises a timer circuit and a secondary winding of transformer T. Both timer circuits comprise a series arrangement of a resistor and a capacitor and interconnect the ends of the respective secondary windings of the transformer T. A junction point of the resistor and the capacitor of each timer circuit is connected, via a further resistor, to the control electrode of an auxiliary transistor whose collector is connected to the control electrode of one of the switching elements in branch A. If a switching element of branch A is in the conducting state, the capacitor of the timer circuit is charged via the resistor of the timer circuit by means of the voltage present between the ends of the secondary winding which is coupled to the conducting switching element. If the voltage across the capacitor is so high that the auxiliary transistor becomes conducting, then the conducting switching element of branch A is rendered non-conducting thereby. Thus, the period of conductance of both switching elements of branch A and hence the frequency of the high-frequency current are determined by the RC periods of the timer circuits. If the lamp operated by means of the switching device is not yet ignited, the current through the load branch is not attenuated, so that the voltages across the secondary windings have a relatively high amplitude. This relatively high amplitude causes the capacitors of the timer circuits to be charged more rapidly so that the switching frequency of the switching elements in branch A increases. This relatively high frequency can be increased further by providing breakdown elements across the resistors of the timer circuits, which breakdown elements become conducting only as a result of the relatively high amplitude of the voltages between the ends of the secondary windings if the lamp is not yet ignited. In the known switching device, these breakdown elements are constructed as zener diodes. In general, the switching device is so dimensioned that an increase of the frequency causes a decrease of the voltage across the lamp and of the current through the electrodes of the lamp. This means that these electrodes are pre-heated by a current which is sufficiently low not to cause damage to the electrodes, while the voltage across the lamp is low enough to preclude that the lamp ignites when its electrodes are not pre-heated. Frequently, the lamp ignites by means of the same voltage after the electrodes have been pre-heated. Dependent upon the dimensions of the switching device and of the lamp, it is sometimes impossible to ignite the lamp by means of the same voltage as that present across the lamp to pre-heat the electrodes or, if pre-heating of the electrodes does not take place, by means of the voltage present across the lamp after the switching device has been put into operation.